Conventionally, a reticle has a cross-like pattern and is used as a sight in a targeting scope, such as a riflescope, to aim at a target. Furthermore, the reticle is used as a scale in an optical instrument, such as a monocular, a binocular, a surveying instrument, and a spotting scope, to determine the size and distance of an observed object.
Such a reticle may be produced by orthogonally crossing two thin wires in a cross-like manner, processing a metal plate or foil to form a cross-like pattern, using a groove or ink to mark a cross-like pattern on a transparent material such as a glass, or by using a half-mirror to project a cross-like pattern.
When an optical instrument having such a reticle is used in the dark, such as at dusk or in the nighttime, or used to aim at a target having a color with low luminosity, such as black, it becomes difficult to view the reticle.
In order to solve such a problem, Japanese Laid-Open Utility Model Publication No. 3-29910 (pages 6–7, FIG. 1) describes directly illuminating a reticle with a light source such as a lamp and projecting an optical image from a light source onto the reticle using a half-mirror. Furthermore, U.S. Pat. No. 1,302,353 (pages 1–2, FIGS. 1–7) describes applying a radium light-emitting substance to a groove formed in a glass plate or applying the radium light-emitting substance around a reticle.
However, when the light source, such as a lamp, is used, a power source, such as a battery, becomes necessary to light the light source in the optical instrument. This increases the size and weight of the optical instrument. Further, the lamp cannot be used when the power source drains.
Furthermore, when a light-emitting material containing radioactive substance, such as the radium light-emitting substance, is used, radioactive exposure to the radioactive substance may cause a problem. In addition, the disposal of such a radioactive substance may cause an environmental problem.
Furthermore, there are phosphorescence light-emitting substances that do not contain radioactive substances, such as zinc sulfide (ZnS), which are known in the related art. However, with zinc sulfide, the emission time after the removal of light, or the afterglow, is between several minutes and several tens of minutes and is thus very short. Therefore, such substances have little practicability.
There is another example that uses fluorescent dye. However, fluorescent dye does not emit light by itself and some sort of a light source becomes necessary. Thus, the same problem as in the example using the light source arises.